1. Field of the Invention
The present invention relates generally to the field of telecommunications systems, and, more particularly, the present invention relates to a subscriber terminal circuit for connecting an analog subscriber line to a digital time-division multiplex telephone switching center.
2. Description of the Related Art
Despite the trend in subscriber terminal circuits of modem digital time-division multiplex telephone exchanges towards systems that are as completely semiconductor-based as possible and the increasing integration of the components, there remain functions which up to now have always been embodied in devices using relays. This is primarily due to technological reasons and reasons of cost. These devices include devices which perform switch off functions in which the leads of the subscriber line are disconnected from the subscriber terminal circuit. A disconnection of this sort is related to the performance feature "Precut over," in which, during installation of a new central office, entire groups of subscriber terminal circuits have to be disconnected from the main distributor. It is also related to the performance feature "power cross," in which, due to the occurrence of impermissibly high interference voltages on a subscriber line, the line is disconnected from the subscriber terminal circuit in order to prevent effects of the over voltages on the semiconductor elements of the subscriber terminal circuit.
In addition, the test access of a testing apparatus to the subscriber terminal circuit and to the leads of the subscriber line also is a function realized by relays. In previous solutions, three separate relays were used for the above-mentioned relay functions. For example, one is used for the disconnection of the leads of the subscriber line in the functions precut over and power cross, and two relays are used in connection with the test access. One of which serves for the connection of the test apparatus to the subscriber terminal circuit with disconnection of the subscriber line from the inputs of the subscriber terminal circuit so that monitoring of the operation of the subscriber terminal circuit can be carried out. The other serves for the connection of the test apparatus to the subscriber line, as well as with disconnection of the subscriber line from the subscriber terminal circuit in order to enable a monitoring of the subscriber line to be carried out.
One goal of the present invention is to reduce the expense for realization of the mentioned relay functions. Other objects and advantages of the present invention will be apparent from the following summary and detailed description of the present invention.